1. Field of the Invention
The present invention relates generally to a method and system for measuring the time-of-flight (TOF) of a radio signal and, more particularly, to a method and system that measures the TOF of a radio signal in a multi-path environment by reducing the amplitude of one or more of the received reflected paths by performing, for example, null steering on an antenna array of the receiving device, to increase the probability of measuring the direct path.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, granted on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, granted on Mar. 29, 2005, the entire content of each being incorporated herein by reference.
In a wireless network, it is important to be able to measure the TOF of radio signals with accuracy. Knowing the TOF of radio signals between the various nodes of the network enables the distances between the nodes to be calculated and, ultimately, allows each of the nodes to be pinpointed in a 3-axis coordinate system. There are some environments in which measuring TOF can be challenging, however. Such environments include those in which the radio signals are often reflected and scattered, such as offices and factories. These environments are often called “multi path” environments, since radio waves can travel multiple paths to get from a source to a receiver—some of which are not direct, but are reflected. Obtaining accurate TOF measurements depends on measuring the TOF of signals that travel along a “direct path.” However, direct path signals may be very weak and difficult to measure, due to scaling by the Automatic Gain Control of the receiver (limited dynamic range) and because of numerical limitations in base band processing.